Add item(s) to multiple collections simultaneously via `add()`

[DmitrySharabin]

So that authors could do the following:

<button mv-action="add(foo, inner)">Add</button>

<div mv-list="outer">
  <div>
    <div mv-list="inner">
      <div></div>
    </div>
  </div>
</div>

[LeaVerou]

This should be fairly easy to implement. If we see an array of items that is not a collection itself, we can see if the items of the array belong to any collections and add to those instead.

One question is: What do we do when some items in an array belong to collections and others do not? Do you add to those collections, or do you give up altogether?